Magnetic memory element having controlled nucleation site in data layer

ABSTRACT

A ferromagnetic data layer of a magnetic memory element is formed with a controlled nucleation site. A Magnetic Random Access Memory (“MRAM”) device may include an array of such magnetic memory elements.

BACKGROUND

[0001] Consider the example of a Magnetic Random Access Memory (MRAM)device. The device includes a resistive cross point array of magnetictunnel junctions. Each magnetic tunnel junction is located at a crosspoint of a word line and a bit line, and has a magnetization thatassumes one of two stable orientations at any given time. These twostable orientations, parallel and anti-parallel, represent logic valuesof ‘0’ and ‘1.

[0002] A write operation on a “selected” magnetic tunnel junction may beperformed by supplying write currents to the word and bit lines crossingthe selected magnetic tunnel junction. The write currents create twoorthogonal external magnetic fields. The magnetic tunnel junctions aredesigned to switch (from parallel to anti-parallel or vice versa) onlyin the presence of the two orthogonal magnetic fields.

[0003] A “half-selected” magnetic tunnel junction lies along only oneline that is supplied with a write current (either a bit line or a wordline). Thus, a half-selected magnetic tunnel junction is exposed to onlyone external magnetic field during a write operation. The magnetictunnel junctions are designed not to switch in the presence of a singlemagnetic field.

[0004] In practice, however, switching distributions of the magnetictunnel junctions in an MRAM array are large, and the switching fields ofnominally similar magnetic tunnel junctions are non-uniform. Somehalf-selected magnetic tunnel junctions switch in the presence of only asingle external magnetic field, and some selected magnetic tunneljunctions do not switch in the presence of two orthogonal magneticfields.

[0005] A write error occurs if the magnetization orientation of aselected magnetic tunnel junction is not switched, or if themagnetization orientation of a half-selected magnetic tunnel junction isinadvertently switched. In a large MRAM array, many write errors canplace a substantial burden on error code correction.

SUMMARY

[0006] According to one aspect of the present invention, a ferromagneticdata layer of a magnetic memory element is formed with a controllednucleation site. Other aspects and advantages of the present inventionwill become apparent from the following detailed description, taken inconjunction with the accompanying drawings, illustrating by way ofexample the principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is an illustration of an MRAM device according to anembodiment of the present invention.

[0008]FIG. 2 is an illustration of a magnetic memory element of the MRAMdevice.

[0009]FIG. 3 is an illustration of a hysteresis curve for the magneticmemory element.

[0010]FIG. 4 is an illustration of an array of data layers in the MRAMdevice.

[0011]FIGS. 5a-5 f are illustrations of data layers having differenttypes and arrangements of controlled nucleation sites.

[0012]FIG. 6 is an illustration of a method of fabricating an MRAMdevice according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0013] As shown in the drawings for purposes of illustration, thepresent invention is embodied in an MRAM device including an array ofmagnetic memory elements. Data layers of the MRAM device have controllednucleation sites. The controlled nucleation sites improve switchingdistribution of the magnetic memory elements, which increasesreliability of writing to the magnetic memory elements.

[0014] Reference is now made to FIG. 1, which illustrates an MRAM device10 including an array 12 of memory cells 14. The memory cells 14 arearranged in rows and columns, with the rows extending in an x-directionand the columns extending in a y-direction. Only a relatively smallnumber of memory cells 14 are shown to simplify the description of thedevice 10. In practice, arrays of any size may be used.

[0015] Word lines 16 extend in the x-direction of the memory cell array12, and bit lines 18 extend in the y-direction of the memory cell array12. There may be one word line 16 for each row of the array 12 and onebit line 18 for each column of the array 12. Each memory cell 14 islocated at a cross point of a word line 16 and bit line 18.

[0016] The MRAM device 10 also includes a read/write circuit (not shown)for performing read and write operations on the memory cells 14. Theread/write circuit senses the resistance states of selected memory cellsduring read operations. The read/write circuit supplies write currentsto selected word and bit lines 16 and 18 during write operations.

[0017] Each memory cell 14 includes at least one magnetic memoryelement. The magnetic memory elements may be magnetic tunnel junctions,giant magneto-resistive (GMR) devices, AMR devices, or any othermagnetic memory device in which a data layer is switched. These devicesinclude ferromagnetic data and reference layers separated by a spacerlayer. If the magnetic memory element is a GMR device, the spacer layeris made of a conductive material such as copper. If the magnetic memoryelement is a magnetic tunnel junction, the spacer layer is an insulatingtunnel barrier made of a material such as Al₂O₃.

[0018] Additional reference is made to FIG. 2, which shows an exemplarymagnetic memory element 50. The exemplary magnetic memory element 50 isa magnetic tunnel junction including a pinned layer 52, a data layer 54,and an insulating tunnel barrier 56 between the pinned and data layers52 and 54. The pinned layer 52 has a magnetization (represented byvector M1) that is oriented in the plane of the pinned layer 52 butfixed so as not to rotate in the presence of an applied magnetic fieldin a range of interest. The data layer 54 has a magnetization(represented by vector M2) that is not pinned. Rather, the magnetizationcan be oriented in either of two directions along an axis (the “easy”axis) lying in the plane of the data layer 54 (one direction is shown insolid, and the other direction is shown in dashed). If the magnetizationvectors (M1 and M2) of the pinned and data layers 52 and 54 point in thesame direction, the orientation of the magnetic tunnel junction is saidto be “parallel”. If the magnetization of the pinned and data layers 52and 54 layers point in opposite directions, the orientation of themagnetic tunnel junction is said to be “anti-parallel.”

[0019] The insulating tunnel barrier 56 allows quantum mechanicaltunneling to occur between the data and pinned layers 54 and 52. Thistunneling phenomenon is electron spin dependent, making the resistanceof the magnetic tunnel junction a function of the relative orientationsof the magnetization of the pinned and data layers 52 and 54. Themagnetization orientation and, therefore, the stored logic value may beread by sensing the resistance state of the magnetic tunnel junction.

[0020] The write currents create magnetic fields about the word and bitlines 16 and 18 crossing the selected memory cell. When combined, thesetwo magnetic fields exceed the coercivity of the data layer and causethe magnetization vector (M1) of the data layer 54 to assume a desiredorientation. A hysteresis curve for the magnetic tunnel junction isshown in FIG. 3. Coercivity is denoted by Hc. When the combined magneticfield exceeds the coercivity, the magnetic tunnel junction can beswitched.

[0021] Reference is now made to FIG. 4, which shows a plurality of datalayers 54 in the array 12 of the MRAM device 10. The data layers 54 havecontrolled nucleation sites 58. The nucleation sites 58 are regionswhere the reversal of magnetization is initiated. They have a lowerswitching threshold relative to the neighboring regions 60 of the datalayer 54. The nucleation sites 58 are controlled in that they have thesame locations in the data layers 54 of all memory cells in the array12. The locations are preferably along edges of the data layers 54, andmore preferably near corners.

[0022] Nucleation, the initiation of switching reversal, occurs at thenucleation sites 58. Since a nucleation site 58 is not fully surroundedby the neighboring region 60 of the data layer 54, magnetic exchangeinteraction between the nucleation site 58 and the neighboring region 60of the data layer 54 is reduced, and only occurs at the boundary of theneighboring region 60 and the nucleation site 58. As a result, switchingreversal always begins at the nucleation site 58, even if theneighboring region 60 contains defects.

[0023] If the nucleation sites 58 are formed along the edges on the datalayers 54, the randomness of nucleation is reduced. Consequently,switching distribution (the distribution of coercivities) of the memorycells 14 in the device 10 is more uniform.

[0024] The nucleation sites 58 may be protrusions from the data layers54 or divets in the data layers 54. The shape of the divets orprotrusions may be circular, elliptical, rectangular, or any othershape.

[0025] Size of the nucleation sites 58 may be between 0.25W and 0.75W,where W is the width of the data layer 54. However, the size of thenucleation sites 58 is not limited to that range. The size of thenucleation sites 58 may be much smaller than W, for example, in therange of 0.05W to 0.1W.

[0026] The nucleation sites 58 may be as thick as, or thicker than, thedata layer 54. Thus the protrusions may be as thick as the data layer54, and the divets may extend through the data layer 54.

[0027] The size and shape of the nucleation sites 58 across the array 12may be uniform. Uniform size and shape across the array 12 shouldimprove uniformity of nucleation energy.

[0028] The data layers 54 are not limited to the nucleation sites 58shown in FIG. 4. Other type and arrangements of nucleation sites areshown in FIGS. 5a-5 f. FIGS. 5a, 5 b, 5 c 5 f show that the nucleationsites 58 may be protrusions instead of divets, FIGS. 5b-5 f shows that adata layer 54 may have more than one nucleation site 58; and FIGS. 5b, 5e and 5 f show that two nucleation sites 58 may be formed at differentedges.

[0029] In FIGS. 5b-5 e the nucleation sites 58 are shown as having asymmetric arrangement on the data layers 54. However, they are not solimited. For example, FIG. 5f shows the nucleation sites 58 have anon-symmetric arrangement. The non-symmetric arrangement may be used tocompensate for an offset in switching fields. For example, one sitemight nucleate at a higher switching field for one direction, and alower switching for the other direction. Thus the offset is balanced outdue to magnetostatics.

[0030] Reference is now made to FIGS. 6, which illustrates thefabrication of a first level of an MRAM device. The fabrication will bedescribed in connection with magnetic tunnel junctions.

[0031] The read/write circuit and other circuits are formed in a siliconsubstrate (110). Bit lines are formed on the substrate (112). A stack ofmagnetic memory element material is deposited (114). A stack formagnetic tunnel junctions may include pinned ferromagnetic layermaterial, insulating tunnel barrier material, and data layer material.The data layer material may be deposited before or after the pinnedlayer material.

[0032] Bits are formed (116). Lithography (e.g., photolithography,e-beam lithography) may be used to define a pattern on the stack, andbits may be formed by a process such as ion milling, chemical etching,drying etching, etc. The patterns include the definitions of thenucleation sites. Thus the nucleation sites (e.g., protrusions, divets)are formed during formation of the bits.

[0033] A hard mask may be used during bit formation to define the bits(including the nucleation sites). An advantage of the hard mask is thatit reduces edge roughness and allows the bits to be formed closertogether.

[0034] Each bit may be milled down to its pinned layer. As a result, anucleation site may also be formed on each pinned layer, as well as eachdata layer.

[0035] Gaps between the bits are filled in with an isolation dielectric(118). Then bit lines are deposited (120).

[0036] Additional levels may be added to the MRAM device. An insulationmaterial such as silicon dioxide is deposited on the last level, and anew level is fabricated by repeating steps 112-120.

[0037] The MRAM device may be used in a variety of applications. Forexample, the MRAM device may be used for long term data storage indevices such as solid state hard drives and digital cameras. It may beused for embedded applications such as extremely fast processors andnetwork appliances.

[0038] The present invention is not limited to the specific embodimentsdescribed and illustrated above. Instead, the invention is construedaccording to the claims that follow.

1. A method of fabricating a magnetic memory element, the methodcomprising forming a ferromagnetic data layer with a controllednucleation site.
 2. The method of claim 1, wherein the nucleation siteis not surrounded by a neighboring region of the data layer.
 3. Themethod of claim 1, wherein the nucleation site has a lower switchingthreshold relative to a neighboring region of the data layer
 4. Themethod of claim 1, wherein the nucleation site is formed at an edge ofthe data layer.
 5. The method of claim 1, wherein the nucleation site isformed at a corner of the data layer.
 6. The method of claim 1, whereinthe nucleation site is one of a divet and a protrusion.
 7. The method ofclaim 1, wherein at least one additional nucleation site is formed onthe data layer.
 8. The method of claim 7, wherein the nucleation siteshave a symmetric arrangement on the data layer.
 9. The method of claim7, wherein the nucleation sites have a non-symmetric arrangement. 10.The method of claim 7, wherein the nucleation sites have a uniform sizeand shape.
 11. The method of claim 1, further comprising formingadditional magnetic tunnel junction layers.
 12. A method of fabricatinga data storage device, the method comprising forming an array offerromagnetic data layers, each layer having first and secondneighboring regions, the first region having a lower switching thresholdthan the second region, the first regions being substantially smallerthan the second regions, the first regions at the same location on thedata layers across the array.
 13. The method of claim 12, wherein thefirst regions are located at corners of the data layers.
 14. The methodof claim 12, wherein the first regions are located at edges of the datalayers.
 15. The method of claim 12, wherein the first regions are eitherdivets in the data layers or protrusion from the data layers.
 16. Themethod of claim 12, wherein each data layer has more than one firstregion.
 17. The method of claim 16, wherein each data layer has asymmetric arrangement of first regions.
 18. The method of claim 16,wherein each data layer has a non-symmetric arrangement of firstregions.
 19. The method of claim 12, wherein the first regions have auniform size and shape across the array.
 20. The method of claim 12,wherein the first regions are formed during bit formation.
 21. Themethod of claim 12, further comprising forming additional magnetictunnel junction layers.
 22. A data storage device comprising a pluralityof magnetic memory elements, each element comprising first means forproviding a reference magnetization, and second means for providing adata magnetization, the second means having first and second regions ofdifferent switching thresholds.
 23. A data storage device comprising aplurality of magnetic memory elements, data layers of the elementshaving controlled nucleation sites.
 24. The device of claim 23, whereinthe nucleation sites are formed at edges of the data layers.
 25. Thedevice of claim 23, wherein the nucleation sites are formed at cornersof the data layers.
 26. The device of claim 23, wherein the nucleationsites are divets in the data layers or protrusions from the data layers.27. The device of claim 23, wherein each data layer has at least oneadditional nucleation site.
 28. The device of claim 27, wherein eachdata layer has a symmetric arrangement of the nucleation sites.
 29. Thedevice of claim 27, wherein each data layer has a non-symmetricarrangement of the nucleation sites.
 30. The device of claim 23, whereinthe nucleation sites have the same locations with respect to the datalayers.
 31. The device of claim 23, wherein each magnetic memory elementfurther includes a reference ferromagnetic layer and an insulatingtunnel barrier.
 32. An MRAM device comprising an array of magnetictunnel junctions, each junction including a data layer having controllednucleation sites, the nucleation sites located at the edges of the datalayers, the nucleation sites having uniform location, size and shapeacross the array.
 33. A magnetic memory device comprising: a data layerhaving at least two protrusions, the protrusions extending from edges ofthe data layer; a reference layer; and a spacer layer between the dataand reference layers.
 34. The device of claim 33, wherein theprotrusions extend at corners of the data layer.
 35. device of claim 33,wherein the protrusions have a symmetric arrangement on the data layer.36. The device of claim 33, wherein the protrusions have non-symmetricarrangement to compensate for an offset in switching fields.
 37. Thedevice of claim 33, wherein the protrusions sites have a uniform sizeand shape.